This invention relates to electrical heating devices and more particularly to elongated, flexible electrical resistance cable heaters which have thermostatic control.
The present invention is directed to inexpensive electrical resistance cable heaters adapted to be wrapped about pipes, buried in seed starting beds and the like. The inexpensive cable heaters of the kind to which the present invention is directed include a relatively thin, flexible heating cable which have elongated, electrically conductive resistance heating elements enclosed in flexible insulators, a thermostat for controlling the energization of the heating elements and an insulated envelope for enclosing the thermostat.
One of the typical uses for such cable heaters is to heat water pipes which are exposed to temperatures which might otherwise freeze and break. For instance, water pipes under house trailers, along outside walls of buildings and in chemical plants are frequently wrapped with cable heaters of this type with the thermostat set to provide current to the cable heater to maintain the water in the pipe above 32.degree. F. to keep it from freezing.
Another typical use for the electrical resistance cable heaters of the present invention is to maintain the soil in seed starting beds at an elevated temperature of about 72.degree.-78.degree. F. in order to promote the quick growth of the seeds and seedlings planted in the bed.
The construction of a typical prior art thermostatic cable heater device is illustrated in FIGS. 1-3 of U.S. Pat. No. 3,851,149, issued Nov. 26, 1974 to William C. Daley. As shown in FIGS. 2 and 3 of that patent, the insulated envelope which surrounds the thermostat also encloses the end portions of the resistance heating elements.
One of the major problems which has been encountered with these thermostatic cable heaters is that the resistance heating elements are frequently shut off by the thermostat even though the temperature of the environment is significantly below the on-off set point of the thermostat. Another major problem takes the form of repeated on and off cycling of the heating elements at temperatures at which they should be continually heating. Due to these problems, the heating cable does not maintain the proper predetermined temperature around the object being heated.
These temperature control problems are believed to originate from the fact that every time the resistance heating elements are energized, they give off heat not only to the surrounding environment but also to the air within the envelope. This causes the temperature of the air within the envelope to significantly exceed that of the surrounding environment and causes the thermostat to prematurely shut off current to the heating elements. Of course, as soon as the resistance heating elements have been shut off for a noticeable length of time, the temperature begins to drop in the envelope and it eventually falls below the on-off set point at which time the circuit is again energized and the cycle begins anew. This latter behavior is believed to be the cause of the constant on-off cycling of the cable heater. The end result of these temperature control problems is that the ability of the cable heater to bring the object to be heated to the proper temperature is severely impaired.